The electronic tuning characteristics of a typical millimeter wave varactor controlled oscillator are shown in FIG. 1. The slope of the VCO tuning curve changes from 100 MHz/volt at -5V to 50 MHz/volt at -15V, this being a variation of 50%. Such a variation is unacceptable for certain applications, for example FMCW radars, which require a variation of no more than 1%. There is therefore a need for an oscillator circuit having substantially linear tuning characteristics.
The operating frequency of a conventional VCO also changes with variations in the case temperature, the effect being to shift the tuning characteristic vertically relative to the varactor voltage axis. For example, as shown in FIG. 1, the tuning characteristics at case temperatures of +45.degree. C. and -25.degree. C. are separated by 400 MHz at a varactor voltage of -10V, representing a drift in operating frequency with temperature of 5.7 MHz/.degree. C. There is therefore a need for an oscillator circuit having significantly reduced temperature drift.